Anisotropic crystal circuit

ABSTRACT

A capacitive arrangement which, in some instances, can be used in combination with one or more resistive elements or other circuit elements to form a distributed circuit, wherein a slab of anisotropic single crystal, such as barium titanate, is used as a substrate upon whose major surfaces the circuit electrodes or elements are mounted. When electrodes are disposed on opposite surfaces of the slab between which an electric field exists and if the crystal slab is mounted so that the C axis is perpendicular to this electric field, the dielectric constant of the crystal slab is a maximum and the capacitance of the capacitor formed by said electrodes and the portion of the crystal slab (dielectric substrate) disposed therebetween is a maximum. On the other hand, the dielectric constant of the portion of the crystal slab disposed between the electrodes, or circuit elements, mounted on the same surface of the slab and arranged parallel to the C axis of the slab will be a minimum and, consequently, the capacitance associated with an electric field existing between these electrodes or elements is a minimum. Alternately, the crystal slab may be replaced by a dielectric member having deposited thereupon a polycrystalline layer comprising several single crystals all oriented with their C axes in the same direction and in the direction of the C axis described above for the single crystal slab.

[451 Aug. 22,1972

[ ANISOTROPIC CRYSTAL CIRCUIT [72] lnventor: Emanuel Gikow, West Long Branch,

[73] Assignee: The United States of America as represented by the Secretary of the Army [22] Filed: July 24, 1970 [21] Appl. No.: 58,019

[52] US. Cl. ..333/70, 333/6, 333/79, 317/242 [51] Int. Cl. ..H03h 7/10, H03h 9/00 [58] Field of Search.....333/70, 70 R, 79, 6; 317/242, 317/256, 258

[56] References Cited UNITED STATES PATENTS 3,480,884 11/1969 Metcalf ..333/6 3,238,429 3/1966 Bomhorst ..317/261 2,637,777 5/1953 Kilby ..333/70 R 3,323,084 5/1967 Glanc ..334/11 2,694,185 11/1954 Kodama ..333/70 R 3,022,472 2/ 1962 Tannenbaum et a1. ..333/18 FOREIGN PATENTS OR APPLICATIONS 743,717 1/1956 Great Britain ..333/70 Primary ExaminerEli Lieberman Assistant Examiner-Saxfield Chatmon, Jr. Attorney--l-larry-M. Saragovitz, Edward J. Kelly, Her bert Berl and Daniel D. Sharp [571 ABSTRACT A capacitive arrangement which, in some instances, can be used in combination with one or more resistive elements or other circuit elements to form a distributed circuit, wherein a slab of anisotropic single crystal, such as barium titanate, is used as a substrate upon whose major surfaces the circuit electrodes or elements are mounted. When electrodes are disposed on opposite surfaces of the slab between which an electric field exists and if the crystal slab is mounted so that the C axis is perpendicular to this electric field, the dielectric constant of the crystal slab is a maximum and the capacitance of the capacitor formed by said electrodes and the portion of the crystal slab (dielectric substrate) disposed therebetween is a maximum. On the other hand, the dielectric constant of the portion of the crystal slab disposed between the electrodes, or circuit elements, mounted on the same surface of the slab and arranged parallel to the C axis of the slab will be a minimum and, consequently, the capacitance associated with an electric field existing between these electrodes or elements is a minimum. Alternately, the crystal slab may be replaced by a dielectric member having deposited thereupon a polycrystalline layer comprising several single crystals all oriented with their C axes in the same direction and in the direction of the C axis described above for the single crystal slab.

Patented Aug. 22, 1972 3,6865% C AXIS J) u 33 INVENTOR,

- EMANUEL GIKOW y. amp

1 9 WM ATTORNEYS ANISOTROPIC CRYSTAL CIRCUIT This invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION In conventional design of capacitive, RC and LC networks in which a polycrystalline dielectric substrate is normally used, the dielectric constant is substantially independent of direction. Consequently, if conductive elements, such as electrodes, resistive elements, or inductive elements are mounted ona common surface of such a dielectric substrate, the capacitive coupling between either adjacent ones of such electrodes, adjacent portions of said elements, or electrodes and said elements, is controlled only by the spacing between said electrodes or elements. In materials of high dielectric strength this coupling is usually detrimental to circuit performance. For example, in the design of a distributed RC filter circuit which may include an elongated serpentine resistive element deposited on the dielectric substrate along an undulating path and shunted at periodic-intervals by the capacitance formed between the resistive element and a common electrode mounted on the opposite surface of the dielectric, the capacitance existing between adjacent portions of the undulating resistive element obviously should be minimized. Another type of circuit may be a delay line in the form of a single elongated strip deposited on one surface of the substrate and having distributed capacitance between the strip and a ground electrode mounted on the opposite surface of the substrate. Here, it may be desirable to minimize capacitance distributed along the length of the strip. In prior circuit design, it has been necessary, in the case of adjacent capacitors, in order to reduce the coupling between adjacent capacitors to satisfactory levels, to increase the spacing between electrodes of adjacent capacitors or else to use two or more discrete capacitors. Likewise, the separation between adjacent portions of one or more circuit elements has tobe increased to reduce the undesired capacitance. In all cases, an increase in size of the resulting circuit becomes necessary.

SUMMARY OF THE INVENTION In accordance with the invention, the problem of undesired capacitance between multi-electroded capacitors and elements of a distributed circuit can be alleviated, without unduly increasing the size of the circuit, if the capacitor dielectric is made in the form of a slab of anisotropic single crystal, such as barium titanate (BaTiO having the C axis thereof properly oriented with respect to the electric fields associated with the circuit elements. Altemately, the capacitance may be in the form of a layer of polycrystalline material disposed on a dielectric slab which layer consists of several single anisotropic crystals, such as barium titanate, all of which have their C axis aligned in the same direction and properly oriented, as in the case of the single crystal slab. Either the single crystal slab, or the properly oriented polycrystalline layer, as the case may be, will be referred to as the crystal. Under certain conditions, a single crystal, such as barium titanate, is anisotropic, that is, the dielectric constant of the single crystal in a direction perpendicular to the C axis of the crystal is much higher than the dielectric constant of the crystal in a direction parallel to this C axis. If electrodes are placed on opposite sides of such a dielectric and the slab is oriented so that its C axis is normal to the electric field produced between these electrodes, the capacitance of the capacitor bounded by said electrodes is relativelyhigh. If, on the other hand, electrodes are provided on the same side of the dielectric slab and aligned in the direction of the C axis of the crystal and an electric field exists between these electrodes, that is, parallel to the C axis, the capacitance between such electrodes is relatively small. This principle can be used in applications where several capacitors are to be mounted on the same slab and the capacitance between capacitors is to be minimized. This principle also can be extended to RC and LC distributed circuits, such as filters, striplines, microstrips and delay lines wherein a resistive or inductive element or elements is deposited on one or both major surfaces of the dielectric slab and a distributed capacitance exists in the portions of the dielectric slab deposited either between the resistive or inductive element and a common electrode mounted on the opposite major surface of the slab or between the resistive elements mounted on opposite major surfaces of the slab, as the case may be. With the electric field applied in the manner already mentioned, the capacitance between various portions of the resistive element which are spaced along the direction of the C axis is minimized. On the other hand, the capacitance between the re-' sistive element on one major surface and the other major surface, or the capacitance between the resistive elements on opposed major surfaces (that is, normal to the major surfaces of the slab) can be made to be at any desired value which is large compared with that along the major surface or surfaces of the slab. This reduces substantially the undesired capacitance between convolutions of a'given resistive or inductive element or between adjacent resistive or inductive elements which allows for more predictable network design and miniaturization.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing an embodiment of the invention using two'adjacent capacitors;

FIG. 2 is a view showing a circuit which can make use of the principles underlying the invention;

FIG. 3 is a view showing a distributed circuit such as shown schematically in FIG. 2; and

FIG. 4 is a view showing a typical capacitive arrangement wherein the crystal slab of FIGS. 1 and 3 is replaced by a polycrystalline layer of several aligned single crystals which may be deposited on a mechanical supporting substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 a dielectric slab 10 is shown which is anisotropic single crystal having a C axis, designated by the arrow. The slab 10 can be made of barium titanate (BaTiO which, at a temperature below the Curie point in a carefully controlled crystal growing process becomes a crystal with the C axis in the direction of the long side of the crystal and with A and B axes aligned along the perpendicular side of the crystal end face. The dielectric slab can be the entire grown crystal or a slab cut to the desired thickness therefrom. A pair of capacitors are shown, by way of example, in FIG. 1. The first of these capacitors is formed by a pair of electrodes and 16 and the portion of the dielectric slab 10 disposed between said electrodes. The electrodes 15 and 16 may be mounted by any of several well known methods such as vacuum deposition upon respective major surfaces 18 and 19 of the slab 10. An electric field can be applied across the electrodes 15 and 16 of the first capacitor by means of an appropriate voltage applied to terminals 21 and 22. The crystal slab 10 is oriented so that this electric field is perpendicular to the C axis of the crystal slab 10. Similarly, the second capacitor will be formed by the electrodes 23 and 24 and the portion of the slab 10 disposed therebetween. An electric field normal to the C axis of the slab 10 can be provided by means of appropriate voltage applied to terminals 26 and 27. This voltage may or may not be the same voltage as applied to terminals 21 and 22. Because the dielectric constant of the crystal slab 10 in a direction normal to the C axis of the crystal is relatively large, the capacitance of the two capacitors can be made quite large; the magnitude of the capacitance, of course, will be inversely proportional to the thickness of the slab 10. A capacitance C,, between the electrodes 15 and 23 on the major surface 18 of the slab 10 normally is undesirable and it should be reduced to a minimum. This capacitance is minimized with the arrangement shown in FIG. 1 wherein the electrodes 15 and 23 of the two capacitors are spaced along the C axes of the single crystal slab 10. This capacitance reduction results from the fact that dielectric constants of the anisotropic crystal slab 10 in a direction of the C axis is much smaller than that perpendicular to said C axis. In some instances, it may be desirable to design electrodes 16 and 24 to form a single electrode, such as a common ground plane. Such an electrode 36 is shown in connection with the device of FIG. 4.

The invention is also applicable for use with distributed circuits, such as the low pass filter circuit shown schematically in FIG. 2, which consists of a series of resistive portions 29a, 29b 29n connected between input terminal 30 and output terminal 31 and shunted at periodic intervals by capacitors 32a 32n to a common terminal 33. A filter also could be made using inductive portions rather than resistive portions. As shown in FIG. 3, the dielectric anisotropy of the crystal slab 10 is used to minimize the interturn capacitance between adjacent convolutions 29 a, 29b etc. of the resistive element 29 which meanders along the major surface 18 of slab 10 in the direction of the C axis of the slab to form a portion of a distributed low pass filter circuit such as indicated schematically in FIG. 2, and having input and output terminals 30 and 31, respectively, and a common ground terminal 33 is connected to a ground electrode 36 mounted on the opposite surface 19 of slab 10, with the slab 10 constituting the distributed capacitance of the filter, corresponding to the capacitive balance 32a 32n of the filter circuit of FIG. 2. In some instances, the resistive element 29 could be replaced by an inductive element. For the same reason as mentioned in connection with the multi-electroded capacitance of FIG. 1, the dielectric constant along the C axis of the slab 10, indicated by the arrow in FIG. 3 is small so that undesirable capacitance between adjacent turns of resistive or inductive element 29 is small, while the dielectric constant perpendicular to the C axis, viz., in the thickness direction of slab 10 is quite high. The principles of the invention can be applied also to any other circuit in which circuit elements are disposed on the surface of a dielectric substrate and between the elements or portions of an element, the capacitance should be kept to a minimum, contrasted with the capacitance between electrodes mounted on opposite surfaces ofthe substrate or slab. In other words, the invention is not to be limited to the low pass filter shown in FIGS. 2 and 3.

In the devices of FIGS. 1 and 2, the slab 10 constitutes a single crystal. An alternate construction is indicated in FIG. 4 in which the single crystal slab 10 of FIG. 1 is replaced by a polycrystalline layer 42. The layer 42 may be mounted in a substrate 40 which can be made of any material which will provide the necessary mechanical support for layer 42. In some instances, this substrate 40 can be omitted. The individual particles making up the polycrystalline layer 42 must themselves be anisotropic single crystals of the same type as that constituting the slab 10 of FIGS. 1 and 3. The polycrystalline layer 42 may be applied under control of an electric field appropriate for aligning the C axes in the same direction, as indicated by the arrow in FIG. 4. Since the individual crystals are relatively small, the layer 42 is shown as a single layer for clarity. The coupling between the first capacitor formed by electrodes 15A and 36 and the portion of the layer 42 between these electrodes and the second capacitor formed by electrodes 23A and 36 and that portion of the layer 42 mounted therebetween is minimized because the dielectric constant of the layer 42 disposed between electrodes 15A and 23A in the direction of the C axis of the various aligned crystal in layer 42 is parallel to any electric field existing in the dielectric layer between these two capacitors, which field is parallel to the C axis of the various crystals. The dielectric constant between electrodes 15A and 36 and that between electrodes 23A and 36, that is, the dielectric constant perpendicular to the C axis of the various crystals in layer 42, is contrastingly high.

While the invention has been described in connection with the specific illustrative embodiments, variations thereof will be apparent to those skilled in the art. For example, a stripline construction could be used instead of the construction illustrated in FIG. 3, in which case, the element 29 would be sandwiched between the crystal slab of FIG. 3 and an electroded crystal slab identical to slab 10. Hence, the scope of the invention should be limited only to the appended claims.

What is claimed is:

1. In combination, an anisotropic crystalline dielectric slab having a pair of opposite major surfaces spaced by the thickness of said slab, said slab having a C axis perpendicular to the thickness direction and a dielectric constant along the C axis greater than the dielectric constant in the thickness direction, an undulatingwircuit element mounted on one of said major surfaces and having a plurality of portions spaced along said C axis whereby the capacitance between said portions is minimized, and at least one circuit member mounted on the other of said major surfaces and combining with said circuit element and the portion of said slab disposed therebetween for providing a distributed capacitance which is large compared with the capacitance between said portions.

2. The combination of claim 1 wherein said circuit element is a resistor.

3. The combination according to claim 1 wherein said circuit element is an inductor. 

2. The combination of claim 1 wherein said circuit element is a resistor.
 3. The combination according to claim 1 wherein said circuit element is an inductor.
 4. The combination of claim 1 wherein said circuit element combines with the distributed capacitance of said slab disposed between various portions of said element and said electrode to form a network.
 5. The combination of claim 4 wherein said circuit element is a resistor.
 6. The combination of claim 4 wherein said circuit element is an inductor. 